Understanding and measurement of muscle tone as related to clinical muscle pain.

Measurable sources of muscle tension include viscoelastic tone, physiological contracture (neither of which involve motor unit action potentials), voluntary contraction, and muscle spasm (which we define as involuntary muscle contraction). The latter two depend on motor unit action potentials to generate the tension. Total muscle tension is most accurately measured as stiffness. Thixotropy of muscle is an ubiquitous and functionally important phenomenon that is not commonly recognized. A clinical pain condition associated with increased muscle tension is tension-type headache, which is largely muscular in origin; it is often caused by myofascial trigger points, but not by a pain-spasm-pain cycle, which is a physiologically and clinically untenable concept. Clinical conditions associated with painful muscle spasm include spasmodic torticollis, trismus, unnecessary muscle tension, nocturnal leg cramps, and stiff-man syndrome.